<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .bg{
            background-color: blue;
        }
        #con{
            width:300px;
            height:200px;
            border:5px solid pink;
            padding:30px;

        }
        #con img{
            width:100%;
        }
    </style>
</head>
<body>
    <div id="con" class="list">
        <img src="1.png" style="width:100%"/>

    </div>
</body>
<script>
    var con=document.getElementsByClassName("list");
    con[0].onmouseover=function(){
        var classStr=this.className;
        var classArr=classStr.split(" ");
        if(classArr.indexOf("bg")>=0){
            return;
        }else{
            this.className=this.className+ " bg";
        }
        
    }
    con[0].onmouseout=function(){
        var classStr=this.getAttribute("class");
        var classArr=classStr.split(" ");
        var index=classArr.indexOf("bg");
        if(index>=0){
            classArr.splice(index,1);
            var str=classArr.join(" ");
            this.setAttribute("class",str);
        }else{
            return;
        }
    }
</script>
</html>